The requirement that grease compositions provide adequate lubrication at high temperature for extended periods of time has become increasingly important. For this reason, grease compositions containing a variety of organic thickening agents, such as those containing multiple uriedo or urea functional groups, have been developed. For example, U.S. Pat. Nos. 3,242,210, 3,243,372, and 3,401,027 disclose polyurea grease thickeners obtained by reacting a three-component reactant mixture comprising a monoamine, a diamine and a diisocyanate, or a monoisocyanate, a diisocyanate and a diamine. As a general rule, the reaction product is comprised of a mixture of urea-containing species of varying chain length and urea content. However, by careful control of reaction variables such as, e.g., the relative quantities of reactants employed, the reaction temperature and the rate and order of reactant mixing, a product may normally be obtained which predominates in one polyurea species. The polyurea reaction is preferably carried out in situ in the grease carrier, and the reaction product may be utilized directly as a grease thickener.
While greases thickened with polyurea thickeners are in many respects superior to older lubricants in severe service application, especially with regard to maintenance of grease consistency at high temperatures, such greases suffer several disadvantages which limit their usefulness under practical service conditions. For example, while polyurea-thickened greases show excellent retention of mechanical properties at high temperature (70.degree. C. or above) and high shear, they tend to soften considerably when subjected to low shear. In fact, the tendency to soften under low shear can be so great that the grease can, when subject to mechanical working under these conditions, undergo a change in penetration grade, e.g., from a No. 2 NLGI penetration grade to a No. 1 NLGI penetration grade. This change in penetration grade under low shear is particularly troublesome since it may occur under practical use conditions when the grease is transferred from the original shipping container or is otherwise stirred or handled. Consequently, normal handling of the grease in making it available to the ultimate consumer may change its consistency to such extent that it is no longer the desired penetration grade for the intended application. While it is true that the change in consistency is reversible in that the softened grease can be subjected to high shear to return the grease to its original consistency, this reversal often requires that the softened grease be shipped back to the formulator for reprocessing.